Wide area differential correction systems (WAD) determine correction data to improve the accuracy, integrity, continuity and/or availability for satellite navigation systems. The most predominant of the satellite navigation systems is the Global Positioning System (GPS).
The GPS is owned and operated by the United States Air Force. It consists of nominally 24 satellites that are uniformly dispersed in orbits around the earth. There are six orbital planes, each containing four satellites. The orbits are 26,560 kilometers in height, have an inclination of 55.degree. and are very nearly circular. The orbits are close to semi-synchronous with a period of 11.967 hours. The ground tracks of the orbits precess by 4 minutes per day.
Each satellite carries several atomic clocks to provide a stable time source for the navigation signal it transmits. The data for a navigation signal to be transmitted by each satellite is uplinked to such satellite typically once per day by the GPS Control Segment. The satellites retransmit the navigation data in a navigation signal using two spread spectrum L-band signals L1 and L2. L1 operates at a frequency of 1575.42 MHZ and is modulated by two binary phase shift key (BPSK) pseudo-random noise (PRN) codes; P (or Y) and C/A. L2 operates at a frequency of 1227.6 MHZ and is modulated by only the C/A code.
The Precision code (P-code) has a chipping rate of 10.23 MHZ. The full P-code has a length of 259 days before it repeats. Different satellites transmit different portions of the P-code, truncated to a length of 7 days. The P-code provides GPS user receivers with a precise indication of the time of transmission of the navigation signal within the week. The P-code can be replaced by the Y-code (used to prevent signal spoofing) at the discretion of the Air Force. The P and Y codes are intended for use by receivers authorized by the Department of Defense that contain appropriate keying material.
The second code is the Coarse/Aquisition code (C/A code). This code is available to all GPS receivers. The C/A code operates at a chipping rate 1/10th that of the P-code, 1.023 MHZ. The C/A code has a length of 1023 chips and repeats every millisecond. The C/A code is used for initial signal acquisition and to provide a navigation signal with lower fidelity than the P and Y codes.
Added to the P (or Y) and C/A codes is navigation data. The navigation data includes the ephemeris and clock information for the transmitting satellite, low accuracy almanac for all of the satellites in the constellation, ionospheric corrections, special messages, etc. This data is added at 50 bps and is different for each GPS satellite. More information on the GPS PRN codes and the GPS navigation message can be found in the GPS Interface Control Document Navstar GPS Space Segment/Navigation User Interfaces, ICD-GPS-200C, available from the U.S. Air Force.
GPS users obtain their time and position by receiving the signals from at least four GPS satellites. Each GPS satellite provides its location (x.sub.sv.sbsb.i, y.sub.sv.sbsb.i,z.sub.sv.sbsb.i) in the navigation message and the time that the message was transmitted from the satellite (t.sub.sv.sbsb.i) is discernable from the signal. From the location of the satellites and the propagation times between the satellites and the user, the user location (x.sub.u, y.sub.u, z.sub.u) and the time t.sub.u can be determined. The propagation delays are measured by the user station generating PN code identical to the transmitted code, and approximately synchronized with the transmitted code and detecting the time difference between the locally generated PN code and the received PN code. There are four equations for the propagation time from each of the four satellites to the user. These four equations contain only four unknowns, the user coordinates, x.sub.u, y.sub.u, z.sub.u, and the clock time t.sub.u. Typically, the user will receive navigation signals from more than four satellites and will use a statistical computation to determine the closest fit of the user position and clock time to the received navigation data. The GPS user navigation solution is described in Global Positioning System, Papers published in Navigation, 1980, Institute of Navigation.
The accuracy of the GPS navigation signal is intentionally degraded by the Air Force by what is referred to as Selective Availability (SA). The effects of SA can be removed from the P and Y codes by authorized receivers with appropriate keying material. SA is intended to degrade the accuracy available to adversaries and terrorists.
Another available satellite navigation system is the Global Satellite Navigation System (GLONASS), operated by the Russian Space Forces. The GLONASS has 24 satellites in three orbital planes with a 64.8.degree. inclination. The orbital height is 19,100 km with a period of 11 hours, 15 minutes. Where GPS operates by using different codes on the same frequency, GLONASS operates with a different frequency on each satellite. The principles of navigating with GLONASS are the same as those for navigating with GPS.
In the future, additional satellite-based navigation systems are expected to be deployed. This invention is applicable to any satellite navigation system. For convenience, the term GNSS will be used to denote GPS, GLONASS, and other satellite-based navigation systems.